The present invention relates generally to diagnostic imaging and, more particularly, to a CT detector having a reflector assembly with low cross-talk and high light output. In addition, the present invention relates to a reflector interstitially disposed between scintillators of a scintillator array that reduces cross-talk to improve CT image quality while simultaneously retaining high light output of the scintillators.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
“Cross-talk” between detector cells of a CT detector is common. “Cross-talk” is generally defined as the communication of data between adjacent cells of a CT detector. Generally, cross-talk is sought to be reduced as cross-talk leads to artifact presence in the final reconstructed CT image and contributes to poor spatial resolution. Typically, four difference types of cross-talk may result within a single CT detector. X-ray cross-talk may occur due to x-ray scattering between scintillator cells. Optical cross-talk may occur through the transmission of light through the reflectors that surround the scintillators. Known CT detectors utilize a contiguous optical coupling layer(s), typically epoxy, to secure the scintillator array to the photodiode array. Cross-talk, however, can occur as light from one cell is passed to another through the contiguous layer. Electrical cross-talk can occur from unwanted communication between photodiodes.
Scintillator arrays typically incorporate a reflector layer or coating between adjacent scintillators to limit cross-talk between the scintillators. Generally, the reflector is formed of a material comprising chromium oxide or other types of optically absorbent material to absorb light transmitting across the separation boundaries between scintillators. Because chromium oxide operates as a good absorbent of light, the relative reflectivity of the reflector is reduced, which in some cases may be as much as 60%. As such, incorporating a reflector layer that includes chromium oxide, or similar material, a tradeoff in CT detector design is made between lower cross-talk and reflectivity. If the reflector layer is fabricated without chromium oxide or other optically absorbent materials, cross-talk between scintillators increases. Simply, implementing optically absorbent materials reduces cross-talk but lowers the reflectivity of the reflector.
Reduced reflectivity degrades low signal performance and increased cross-talk affects spatial resolution. Low signal performance is a function of noise generated in the CT detector. As reflectivity falls, the light output of the scintillator also falls. Noise, however, is relatively constant, therefore, decreases in light output increases the ratio of noise to functional light output. Additionally, the amount of cross-talk that may be attributed to scattered x-rays can be estimated to be about 50% of the total cross-talk in the CT detector. While the optically absorbent material is effective in reducing cross-talk associated with the transference of light between scintillators, the reflector typically has poor x-ray absorption characteristics and as such, does not eliminate the x-ray caused cross-talk that may occur between scintillators.
Therefore, it would be desirable to design a CT detector with reduced light and x-ray cross-talk characteristics to improve CT image quality without a sacrifice in light output for improved signal.